1. Field of the Invention
The invention relates to the production of printed circuit boards and deals with foil printed circuit boards with metal platings in etched through or blind holes and a method for the production of such boards or the semifinished products used in their production.
2. Background of the Invention
In the production of thin, multilayer foil circuit boards it has hitherto been necessary to carry out two separate structurings by means of photochemical methods, namely the structuring of through and blind holes, which have to be subsequently plated on, and the structuring of the current paths and pads, which run in electrically conducting layers of foil circuit boards and in plated-on holes.
Although said structurings take place according to known, proven photochemical methods, which are widely used in electrical engineering, there are fundamental disadvantages in such a two-stage production process. Two such structurings are more than twice as expensive as one, photomasks must be produced for each of the said structurings, which require precise positioning and also photoresist layers must be applied to the preliminary product and then removed again.
Two structurings lead to lower outputs than one alone. The total output of two sequentially performed production processes is formed from the product of the individual outputs, i.e. the wastage in the first production process limits the output in the following process.
Two such structurings give rise to extra production costs, which limits the potential physical and electronic possibilities of the photochemical method used.
The production tolerances cause such a restriction. Thin, multilayer foil circuit boards with thicknesses below 100 .mu.m and through and blind holes with a diameter below 100 .mu.m must in the case of two exposures with different photomasks for said two structurings have certain production tolerances, which rapidly reach the same order of magnitude as the actual structures.
The thicknesses of the current paths and the metal platings give rise to such a restriction. Following a structuring of the through and blind holes, the latter e.g. undergo plasma etching and are e.g. plated on with copper. The resulting metal platings in the through and blind holes deteriorate the dimensional stability of the foil circuit board, because electrodeposited metal layers are not strain-free. Galvanic plating leads to an increase in the thickness of the top metal foil of the foil circuit board. The thicker a metal layer, the more difficult it is to chemically etch. The physical advantage of very small dimensions on foil circuit boards is consequently limited, as is the electronic advantage of thin current paths.
It is desirable to have a method which would eliminate these disadvantages of double structuring and which would make it possible to obtain thin, light and rapidly etchable metal layers with limited effort, low costs, low wastage and good dimensional stability. Known and proven methods, materials, etc. are to be used. For example, for structuring the conductive pattern known methods must be usable. The method must also be compatible with respect to the further processing and with varying dimensions of components to be assembled using known methods. Finally only one photochemical structuring must be needed for producing one or two layers of current paths and metal platings in through and blind holes of foil circuit boards.